Ah! So, I had forgotten that base def rolls are always max+1 on non-normal items. I knew that upgrading re-rolls base def, but forgotten how uniques work. Arreat Summit still lists the base range (but does on another page explain this mechanism).
So, yes, that makes sense. Since I was only searching for references to 507 def HoZ, I only found a handful. You're right that there are many more 200ED HoZs around. And, yes. Ohm appears to be the going rate for 200ED.